The developer, who goes by the alias of Orbit84, said: “I am sincerely sorry about this and hope that we will get the funds back.

“I am in talks with my hosting provider to get as much information about the hacker and will see what can be done with it.

“If you ever entered your key on BlackWallet, you may want to move your funds to a new wallet.”

What are crypto-currencies?

A crypto or virtual currency is a type of money that is completely digital. You can use currencies such as Bitcoin to buy products and services, but not many shops accept them yet. In many countries they are heavily regulated.

Owners typically keep their currency in digital wallets and can transfer payments to other wallets fairly anonymously, without a bank or broker being involved.

Transactions are recorded in a public list called a blockchain – a type of ledger which both records and verifies who spent or transferred which coins. The blockchain exists on computers that solve cryptographic problems as they log transactions to help combat fraud.

Stellar is an open-source payments network focused on reducing poverty in the developing world. It says its lumens – which work using blockchain technology – make monetary transactions cheaper, quicker and more reliable.

According to CoinMarketCap, a site that ranks the value of different crypto-currencies, there is $11.6bn worth of Stellar Lumen in circulation.

That is far less than the top virtual currencies Bitcoin and Ethereum, which are thought to be worth $240.3bn and $127.4bn respectively.

In 2016, the price of bitcoin fell more than 10% after the Hong Kong-based digital currency exchange Bitfinex said it had suffered a major hack.

The security breach led to the possible theft of bitcoin worth $65m (£49m).

And in 2017 hackers targeted the Bithumb exchange in South Korea, taking millions of dollars’ worth of Bitcoin and Ethereum.

The personal information of 30,000 people was also stolen in the attack, which was credited to North Korea.